This invention relates to a method of and means for automatically regulating the extraction of sludge from a waste water or sewage treatment apparatus, notably a sludge-bed type settling tank.
As a rule, sludge is concentrated in a so-called concentrator sludge pit. Sludge can be removed therefrom by using continuously-operating or intermittently-operating devices. It is known to adjust the rate and duration of these sludge extractions or purifying steps, by using means responsive to the input of sewage or waste water introduced into the apparatus and or to a preset sludge extraction rate. The sludge accumulation is not at all times exactly proportional to the throughput of water to be treated, for it also depends on the specific nature of this water, i.e. the nature of the sludge. Therefore, the sludge level in the concentrator must be checked continuously. This level is usually controlled by means of a densimeter or a capacitive or ultrasonic probe. In addition, it is known that above the sludge level there is a region in which the water muddiness is more or less preserved, as a function of the specific nature of the suspended solids therein, hence of the sludge type. Therefore, it is extremely difficult to determine the precise upper level of the sludge. Using a densimeter or hydrometer requires frequent adjustments. Besides, the results obtained by using probes are inaccurate, since probes do not take into account this region of diffused cloudiness or turbidity which varies with sudden changes in the waste water input supplied to the apparatus. As a consequence of these delicate and necessarily inaccurate measurements of the sludge level, the extraction rate and duration are such that, since they do not take into account the level variations, either an excessive extraction with attendant loss of water and costly reagents, or an insufficient extraction with sludge overflow and an increase in the turbidity of the treated water, are observed.